GENERATING GABAergic NEURONS IN BRAINS

ABSTRACT

This document provides methods and materials for generating GABAergic neurons in brains. For example, methods and materials for using nucleic acid encoding a NeuroD1 polypeptide and nucleic acid encoding a Dlx2 polypeptide to trigger glial cells (e.g., NG2 glial cells or astrocytes) within the brain (e.g., striatum) into forming GABAergic neurons (e.g., neurons resembling medium spiny neurons such as DARPP32-positive GABAergic neurons) that are functionally integrated into the brain of a living mammal (e.g., a human) are provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 15/436,275, filed Feb. 17, 2017, which claims the benefit of U.S. Provisional Application Ser. No. 62/296,960, filed Feb. 18, 2016. The disclosures of the prior applications are considered part of (and are incorporated by reference in) the disclosure of this application.

STATEMENT AS TO FEDERALLY SPONSORED RESEARCH

This invention was made with government support under Grant Nos. AG045656 and MH083911, awarded by the National Institutes of Health. The Government has certain rights in the invention.

BACKGROUND 1. Technical Field

This document relates to methods and materials for generating GABAergic neurons in brains. For example, this document relates to methods and materials for using nucleic acid encoding a NeuroD1 polypeptide and nucleic acid encoding a Dlx2 polypeptide to trigger glial cells (e.g., NG2 glial cells or astrocytes) within the brain (e.g., striatum) into forming GABAergic neurons (e.g., neurons resembling parvalbumin neurons or medium spiny neurons such as DARPP32-positive GABAergic neurons) that are functionally integrated into the brain of a living mammal (e.g., a human).

2. Background Information

Huntington's disease is mainly caused by mutations in the gene huntingtin (HTT), resulting into the expansion of trinucleotide CAG repeats that encode polyglutamine. When the number of CAG repeats in a huntingtin gene exceeds 36, it will cause disease, and the GABAergic medium spiny neurons in the striatum are in particular vulnerable to such polyglutamine toxicity (Ross et al., Lancet Neurol., 10:83-98 (2011); and Walker, Lancet, 369:218-228 (2007)). Currently, there is no effective treatment to cure Huntington's disease.

SUMMARY

This document provides methods and materials for generating GABAergic neurons in brains. For example, this document provides methods and materials for using nucleic acid encoding a NeuroD1 polypeptide and nucleic acid encoding a Dlx2 polypeptide to trigger glial cells (e.g., NG2 glial cells or astrocytes) within the brain (e.g., striatum) into forming GABAergic neurons (e.g., neurons resembling parvalbumin neurons or medium spiny neurons such as DARPP32-positive GABAergic neurons) that are functionally integrated into the brain of a living mammal (e.g., a human).

As described herein, nucleic acid designed to express a NeuroD1 polypeptide and nucleic acid designed to express a Dlx2 polypeptide can be delivered together to glial cells (e.g., NG2 glial cells or astrocytes) within a mammal's brain (e.g., striatum) in a manner that triggers the glial cells to form functional and integrated GABAergic neurons. These functional and integrated GABAergic neurons can resemble medium spiny neurons (e.g., they can be DARPP32-positive GABAergic neurons). Having the ability to form new GABAergic neurons within the striatum of a living mammal's brain using the methods and materials described herein can allow clinicians and patients (e.g., Huntington's disease patients) to create a brain architecture that more closely resembles the architecture of a healthy brain when compared to the architecture of an untreated Huntington's disease patient's brain following the significant death or degeneration of GABAergic medium spiny neurons. This can represent an important step forward for Huntington's disease patients even though there is currently no cure for the disease. In some cases, having the ability to replenish GABAergic medium spiny neurons within the striatum that die or degenerate during Huntington's disease progression using the methods and materials described herein can allow clinicians and patients to slow, delay, or reverse Huntington's disease progression.

In general, one aspect of this document features a method for forming GABAergic neurons in a striatum of a living mammal's brain. The method comprises, or consists essentially of, administering nucleic acid encoding a NeuroD1 polypeptide and nucleic acid encoding a Dlx2 polypeptide (or a NeuroD1 polypeptide and a Dlx2 polypeptide) to glial cells within the striatum, wherein the NeuroD1 polypeptide and the Dlx2 polypeptide are expressed by the glial cells, and wherein the glial cells form or are converted into GABAergic neurons within the striatum. The mammal can be a human. The glial cells can be NG2 glial cells or astrocytes. The GABAergic neurons can be parvalbumin-positive or DARPP32-positive. The NeuroD1 polypeptide can be a human NeuroD1 polypeptide. The Dlx2 polypeptide can be a human Dlx2 polypeptide. The nucleic acid encoding the NeuroD1 polypeptide can be administered to the glial cells in the form of a viral vector. In such cases, the viral vector can be an adeno-associated viral vector (e.g., an adeno-associated virus serotype 2 viral vector, an adeno-associated virus serotype 5 viral vector, or an adeno-associated virus serotype 9 viral vector). The nucleic acid encoding the Dlx2 polypeptide can be administered to the glial cells in the form of a viral vector. In such cases, the viral vector can be an adeno-associated viral vector (e.g., an adeno-associated virus serotype 2 viral vector, an adeno-associated virus serotype 5 viral vector, or an adeno-associated virus serotype 9 viral vector). The nucleic acid encoding the NeuroD1 polypeptide and the nucleic acid encoding the Dlx2 polypeptide can be located on the same viral vector, and the viral vector can be administered to the glial cells. In such cases, the viral vector can be an adeno-associated viral vector (e.g., an adeno-associated virus serotype 2 viral vector, an adeno-associated virus serotype 5 viral vector, or an adeno-associated virus serotype 9 viral vector). The nucleic acid encoding the NeuroD1 polypeptide and the nucleic acid encoding the Dlx2 polypeptide can be located on separate viral vectors, and each of the separate viral vectors can be administered to the glial cells. In such cases, each of the separate viral vectors can be an adeno-associated viral vector (e.g., an adeno-associated virus serotype 2 viral vector, an adeno-associated virus serotype 5 viral vector, or an adeno-associated virus serotype 9 viral vector). The administration can comprise a direct injection into the striatum of the living mammal's brain. The administration can comprise an intraperitoneal, intracranial, intravenous, intranasal, or oral administration. The nucleic acid encoding the NeuroD1 polypeptide can be operably linked to a promoter sequence; and the promoter sequence can be constitutive promoter sequence. The constitutive promoter sequence can comprise a NG2 promoter sequence, a GFAP promoter sequence, an EF1a promoter sequence, a CMV promoter sequence, an Aldh1L1 promoter sequence, or a CAG promoter sequence. The nucleic acid encoding the NeuroD1 polypeptide can be operably linked to a promoter sequence; and the promoter sequence can be a glial-specific promoter sequence. The glial-specific promoter sequence can comprise a NG2 promoter sequence, a GFAP promoter sequence, an Aldh1L1 promoter sequence, or an Olig2 promoter sequence. The nucleic acid encoding the Dlx2 polypeptide can be operably linked to a promoter sequence; and the promoter sequence can be constitutive promoter sequence. The constitutive promoter sequence can comprise a NG2 promoter sequence, a GFAP promoter sequence, an EF1a promoter sequence, a CMV promoter sequence, an Aldh1L1 promoter sequence, or a CAG promoter sequence. The nucleic acid encoding the Dlx2 polypeptide can be operably linked to a promoter sequence; and the promoter sequence can be a glial-specific promoter sequence. The glial-specific promoter sequence can comprise a NG2 promoter sequence, a GFAP promoter sequence, an Aldh1L1 promoter sequence, or an Olig2 promoter sequence.

In another aspect, this document features a composition for forming GABAergic neurons in a striatum of a living mammal's brain. The composition comprises, or consists essentially of, a nucleic acid vector comprising a nucleic acid sequence encoding a NeuroD1 polypeptide and a nucleic acid sequence encoding a Dlx2 polypeptide. The nucleic acid vector can be a viral vector such as an adeno-associated viral vector (e.g., an adeno-associated virus serotype 2 viral vector, an adeno-associated virus serotype 5 viral vector, or an adeno-associated virus serotype 9 viral vector). The nucleic acid sequence encoding the NeuroD1 polypeptide can be operably linked to a promoter sequence; and the promoter sequence can be constitutive promoter sequence. The constitutive promoter sequence can comprise a NG2 promoter sequence, a GFAP promoter sequence, an EF1a promoter sequence, a CMV promoter sequence, an Aldh1L1 promoter sequence, or a CAG promoter sequence. The nucleic acid sequence encoding the NeuroD1 polypeptide can be operably linked to a promoter sequence; and the promoter sequence can be a glial-specific promoter sequence. The glial-specific promoter sequence can comprise a NG2 promoter sequence, a GFAP promoter sequence, an Aldh1L1 promoter sequence, or an Olig2 promoter sequence. The nucleic acid sequence encoding the Dlx2 polypeptide can be operably linked to a promoter sequence; and the promoter sequence can be constitutive promoter sequence. The constitutive promoter sequence can comprise a NG2 promoter sequence, a GFAP promoter sequence, an EF1a promoter sequence, a CMV promoter sequence, an Aldh1L1 promoter sequence, or a CAG promoter sequence. The nucleic acid sequence encoding the Dlx2 polypeptide can be operably linked to a promoter sequence; and the promoter sequence can be a glial-specific promoter sequence. The glial-specific promoter sequence can comprise a NG2 promoter sequence, a GFAP promoter sequence, an Aldh1L1 promoter sequence, or an Olig2 promoter sequence.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.

Other features and advantages of the invention will be apparent from the following detailed description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIGS. 1A-H. Conversion of cultured NG2 cells into functional GABAergic neurons. (A) Differentiation of NG2 cells infected by control retrovirus (expressing GFP under NG2 promoter) into immature oligodendrocytes (CNPase-positive, red) after 3 days post-infection (DPI). (B) NG2 cells infected by NG2::Dlx2 retrovirus were reprogrammed into neurons (NeuN-positive, red, 7 DPI). (C) NG2-converted neurons were innervated by GABAergic synapses, as shown by GABAergic presynaptic protein GAD65 (red, 14 DPI). (D) Representative traces recorded from NG2-converted neurons showing upward spontaneous synaptic events when holding at −20 mV (14 DPI). Note all events were blocked by the GABA_(A) receptor antagonist BIC (20 μM), suggesting that they were GABAergic events. (E-F) NeuroD1 enhanced the conversion efficiency induced by Dlx2, as shown by Tuj1 staining (E, 14 DPI) and GAD67 staining (F, 21 DPI). (G-H) Quantified data showing a significant increase of the number of Tuj1 positive neurons (G) or GAD67 positive GABAergic neurons (H) after coexpressing NeuroD1 with Dlx2 together. Data were presented as mean±s.e.m. ***P<0.001 (Student's t-test). Scale bars: 40 μm for panels A, B, E, and F; 10 μm for panel C.

FIGS. 2A-D. Characterizing mouse NG2 cultures. (A-C) Infecting mouse NG2 cultures with control retrovirus NG2::GFP revealed a small percentage of cells immunopositive for astrocyte marker GFAP, but not microglia marker Iba1 or immature neuron marker DCX. Scale bar: 40 μm. (D) Quantified data showing the majority of NG2 cells will differentiate into oligodendrocytes (CNPase) (3 days after infection of NG2::GFP in differentiation medium).

FIG. 3. No glutamatergic neurons generated from NG2 cells after expressing Dlx2. NG2-converted neurons were immunonegative for vGlut1, suggesting no glutamatergic neurons after reprogramming NG2 cells by Dlx2 alone (n=60 cells in 4 repeats). Scale bar: 40 μm.

FIGS. 4A-D. Screening transcriptional factors for efficient conversion of NG2 cells into GABAergic neurons. (A) GAD67-positive neurons converted from NG2 cells after infection with different combinations of neural transcription factors (Dlx2, NeuroD1, and Ascl1; 14 DPI). (B) Quantified data showing a high conversion efficiency of NG2 cells into GABAergic neurons in Dlx2+NeuroD1 and Dlx2+NeuroD1+Ascl1 groups. (C) Representative traces showing upward GABAergic events recorded from NG2-converted neurons after infection with different transcription factors (21 days post infection). (D) Quantification of the frequency of GABAergic events also showed a high conversion efficiency of NG2 cells into GABAergic neurons by Dlx2+NeuroD1.

FIGS. 5A-H. Classification of NG2-converted GABAergic neurons in culture. (A-E) Immunostaining of NG2-converted neurons with a series of interneuron subtype markers (CR, SST, PV, CCK8 and NPY) after infection with NeuroD1+Dlx2 retroviruses (21 DPI). Scale bars: 40 μm. (F) Quantification showing that many NG2-converted neurons were immunopositive for calretinin (CR), somatostatin (SST), and parvalbumin (PV), but much less for CCK8 or NPY. (G-H) Representative traces showing different action potential firing patterns among NG2-converted neurons (G, 12.9 Hz; H, 38.9 Hz). Note that panel H shows an example of fast-spiking like firing pattern.

FIGS. 6A-H. In vivo reprogramming NG2 cells into functional GABAergic neurons. (A) Schematic diagram showing Cre-mediated FLEx switch of the NeuroD1/Dlx2-P2A-mCherry system. (B) NG2 cells detected after in vivo injection of NG2::Cre and FLEx-mCherry AAV into the striatum. (C) Macroscopic view of AAV-infected striatal region (21 DPI). (D) NeuroD1/Dlx2-infected NG2 cells showed neuron-like morphology and NeuN staining (arrow, red) at 21 days post AAV injection. (E) Quantified data showing a gradual decrease of NG2 cells among infected cells, accompanied by an increase of neurons after NeuroD1/Dlx2 infection, indicating a conversion of NG2 cells into neuronal cells. (F-G) NG2-converted neurons (21 DPI) in the striatum were immunopositive for GABA (F) and GAD67 (G). Scale bars: 40 μm for panel B, D; 20 μm for panel F, H. (H) Spontaneous synaptic events recorded from NG2-onverted neurons (31 DPI).

FIGS. 7A-C. Reprogramming NG2 cells into neurons under direct control of NG2 promoter. (A) NG2 cells revealed by infection of control virus NG2::GFP in the striatum. (B) NG2 cells became NeuN-positive neurons after infection by NG2::NeuroD1/Dlx2. Scale bar: 40 μm. (C) Quantified data showing the percentage of neurons versus NG2 cells after infection by NG2::GFP or NG2::NeuroD/Dlx2. The majority of NG2 cells converted into neurons after expressing NeuroD1+Dlx2 under the direct control of NG2 promoter.

FIGS. 8A-G. Characterizing the subtypes of NG2-converted neurons in striatum. (A-E) Immunostaining with a series of GABAergic neuron subtype markers (SST, PV, NPY, CCK8 and DARPP32) in the striatum after ectopic expression of NeuroD1 and Dlx2 in NG2 cells (21 DPI). Scale bars: 40 μm. (F) Quantified data showing a significant proportion of neurons immunopositive for DARPP32 and PV after NeuroD1/Dlx2 AAV injection into the striatum. (G) Representative traces showing action potential firing patterns recorded from NG2-converted neurons (31 DPI) in brain slices (n=20 neurons). Some neurons showed fast-spiking like firing pattern, with a frequency range of 70-200 Hz.

FIGS. 9A-F. In situ reprogramming cortical NG2 cells into functional GABAergic neurons. (A) NG2 cells revealed after injecting NG2::Cre and FLEx-mCherry AAV into mouse prefrontal cortex. (B) Low-magnification images showing AAV-infected site in the prefrontal cortex (21 DPI). (C) Reprogramming cortical NG2 cells into NeuN-positive neurons after ectopic expression of NeuroD1 and Dlx2 in NG2 cells (21 DPI). (D-E) Some NG2-converted neurons were immunopositive for GABA (D) and GAD67 (E) in the striatum (21 DPI). Scale bars: 40 μm for panels A, C; 500 μm for panel B; 20 μm for panels D, E. (F) Representative trace showing spontaneous synaptic events recorded from in situ NG2-converted neurons in the prefrontal cortex (35 DPI).

FIGS. 10A-G. Characterizing subtypes of NG2-converted neurons in prefrontal cortex. (A-E) Immunostaining showing different subtypes of GABAergic neurons among NG2-converted cells after NeuroD1/Dlx2 infection in the prefrontal cortex (21 DPI). Scale bars: 40 μm. (F) Quantified data showing a significant number of NG2-converted neurons in the prefrontal cortex being immunopositive for PV and CCK8. (G) Representative traces showing low and high frequency action potential firing patterns among NG2-converted neurons (35 DPI) in the prefrontal cortex (n=8 neurons). Note some neuron showed fast-spiking like action potential firing (138 Hz).

FIGS. 11A-B. Control virus infected mainly NG2 cells in the mouse brain (A) Control AAV (NG2::Cre and FLEx-mCherry) infected mostly NG2 cells after injection into the striatum. (B) Control AAV (NG2::Cre and FLEx-mCherry) also infected mainly NG2 cells in the prefrontal cortex. Scale bar: 40 μm.

FIG. 12 is a listing of an amino acid sequence of a human NeuroD1 polypeptide (SEQ ID NO:1).

FIG. 13 is a listing of an amino acid sequence of a human Dlx2 polypeptide (SEQ ID NO:2).

FIGS. 14A-F. NeuroD1 and Dlx2 mediate glia-to-neuron conversion in the striatum of R6/2 mice, a mouse model for Huntington's disease. (A, B) R6/2 mice were injected with AAV5 viruses expressing NeuroD1 and Dlx2 or mCherry control in astrocytes under the Cre-FLEx system. After 1 month, NeuroD1 (arrows) and Dlx2 (arrows) were detected in the infected cells (arrows) in NeuroD1/Dlx2 injected mice (B), but not in the mCherry control mice (A). (C) In the mCherry control group, the majority of infected cells were astrocytes labeled by GFAP (arrows). (D) In the NeuroD1/Dlx2 group, mCherry-positive cells were not co-localized with GFAP, but some exhibited a neuron-like morphology. (E, F) The NeuroD1/Dlx2-mediated glia-to-neuron conversion in R6/2 mouse striatum was confirmed by NeuN staining (arrows). The Htt aggregations in nucleus (co-localized with DAPI) were observed in both groups, confirming they were Huntington's disease mouse model mice.

FIGS. 15A-B. Characterization of glia-converted neurons in the R6/2 mouse striatum. (A) NeuroD1/Dlx2-mediated glia-converted neurons (35 DPI) in R6/2 mouse striatum were immuno-positive for GAD67 (arrows) and GABA (arrows). (B) Some of the converted neurons also were labeled with DARPP32 (arrows), a marker for striatal medium spiny GABAergic neurons, demonstrating that the glia-converted neurons can replenish the lost DARPP32 neurons in Huntington's disease.

FIGS. 16A-D. NeuroD1 and Dlx2 expressed using AAV9 viral vectors mediate glia-to-neuron conversion in the striatum of R6/2 mice, a mouse model for Huntington's disease. (A, B) AAV9 was injected into the striatum, and Dlx2 and NeuroD1 were detected in the infected cells at 10 days post injection (arrows). (C) In the mCherry control group, the majority of infected cells were co-localized with astrocytic marker GFAP (arrows). (D) In the NeuroD1/Dlx2 group, most of mCherry positive cells were co-localized with neuronal marker NeuN (arrows).

FIG. 17A-B. Characterization of glia-converted neurons in the striatum by AAV9. (A) NeuroD1/Dlx2 converted neurons were immuno-positive for GABAergic neuronal marker GAD67 (arrows). (B) Some of the converted neurons also were immuno-positive for DARPP32 (arrows), a marker for striatal GABAergic medium spiny neurons.

DETAILED DESCRIPTION

This document provides methods and materials for generating GABAergic neurons in brains. For example, this document provides methods and materials for using nucleic acid encoding a NeuroD1 polypeptide and nucleic acid encoding a Dlx2 polypeptide to trigger glial cells within the brain into forming GABAergic neurons that can be functionally integrated into the brain of a living mammal. Forming GABAergic neurons as described herein can include converting glial cells within the brain into GABAergic neurons that can be functionally integrated into the brain of a living mammal. In some cases, the methods and materials described herein can be used to improve the brain architecture of Huntington's disease patient's brain such that it more closely resembles the brain architecture of a healthy human, to restore a healthy brain architecture to a Huntington's disease patient's brain, to reduce the progression of Huntington's disease, to delay the onset of Huntington's disease symptoms, and/or to treat Huntington's disease. In some cases, the methods and materials described herein can be used to reverse the effects of Huntington's disease in a mammal with Huntington's disease.

Any appropriate mammal can be treated as described herein. For example, mammals including, without limitation, humans, monkeys, dogs, cats, cows, horses, pigs, rats, and mice can be treated as described herein to generate GABAergic neurons in the brain of a living mammal. In some cases, a human having Huntington's disease can be treated as described herein to generate GABAergic neurons in a Huntington's disease patient's brain. A mammal can be identified as having Huntington's disease using any appropriate Huntington's disease diagnostic technique. For example, a genetic screen of the Huntingtin gene can be performed to diagnose a human as having Huntington's disease.

As described herein, a mammal can be treated by administering nucleic acid designed to express a NeuroD1 polypeptide and nucleic acid designed to express a Dlx2 polypeptide to glial cells (e.g., NG2 glial cells or astrocytes) within the mammal's brain (e.g., striatum) in a manner that triggers the glial cells to form functional and integrated GABAergic neurons. Examples of NeuroD1 polypeptides include, without limitation, those polypeptides having the amino acid sequence set forth in GenBank® accession number NP_002491 (GI number 121114306). A NeuroD1 polypeptide can be encoded by a nucleic acid sequence as set forth in GenBank® accession number NM_002500 (GI number 323462174). Examples of Dlx2 polypeptides include, without limitation, those polypeptides having the amino acid sequence set forth in GenBank® accession number NP_004396 (GI number 4758168). A Dlx2 polypeptide can be encoded by a nucleic acid sequence as set forth in GenBank® accession number NM_004405 (GI number 84043958).

Any appropriate method can be used to deliver nucleic acid designed to express a NeuroD1 polypeptide and nucleic acid designed to express a Dlx2 polypeptide to glial cells within the brain of a living mammal. For example, nucleic acid encoding a NeuroD1 polypeptide and nucleic acid encoding a Dlx2 polypeptide can be administered to a mammal using one or more vectors such as viral vectors. In some cases, separate vectors (e.g., one vector for nucleic acid encoding a NeuroD1 polypeptide, and one vector for nucleic acid encoding a Dlx2 polypeptide) can be used to deliver the nucleic acids to glial cells. In some cases, a single vector containing both nucleic acid encoding a NeuroD1 polypeptide and nucleic acid encoding a Dlx2 polypeptide can be used to deliver the nucleic acids to glial cells.

Vectors for administering nucleic acids (e.g., nucleic acid encoding a NeuroD1 polypeptide and nucleic acid encoding a Dlx2 polypeptide) to glial cells can be prepared using standard materials (e.g., packaging cell lines, helper viruses, and vector constructs). See, for example, Gene Therapy Protocols (Methods in Molecular Medicine), edited by Jeffrey R. Morgan, Humana Press, Totowa, N.J. (2002) and Viral Vectors for Gene Therapy: Methods and Protocols, edited by Curtis A. Machida, Humana Press, Totowa, N.J. (2003). Virus-based nucleic acid delivery vectors are typically derived from animal viruses, such as adenoviruses, adeno-associated viruses, retroviruses, lentiviruses, vaccinia viruses, herpes viruses, and papilloma viruses. In some cases, nucleic acid encoding a NeuroD1 polypeptide and nucleic acid encoding a Dlx2 polypeptide can be delivered to glial cells using adeno-associated virus vectors (e.g., an adeno-associated virus serotype 2 viral vector, an adeno-associated virus serotype 5 viral vector, or an adeno-associated virus serotype 9 viral vector), lentiviral vectors, retroviral vectors, adenoviral vectors, herpes simplex virus vectors, or poxvirus vector.

In addition to nucleic acid encoding a NeuroD1 polypeptide and/or nucleic acid encoding a Dlx2 polypeptide, a viral vector can contain regulatory elements operably linked to the nucleic acid encoding a NeuroD1 polypeptide and/or a Dlx2 polypeptide. Such regulatory elements can include promoter sequences, enhancer sequences, response elements, signal peptides, internal ribosome entry sequences, polyadenylation signals, terminators, or inducible elements that modulate expression (e.g., transcription or translation) of a nucleic acid. The choice of element(s) that may be included in a viral vector depends on several factors, including, without limitation, inducibility, targeting, and the level of expression desired. For example, a promoter can be included in a viral vector to facilitate transcription of a nucleic acid encoding a NeuroD1 polypeptide and/or a Dlx2 polypeptide. A promoter can be constitutive or inducible (e.g., in the presence of tetracycline), and can affect the expression of a nucleic acid encoding a polypeptide in a general or tissue-specific manner. Examples of tissue-specific promoters that can be used to drive expression of a NeuroD1 polypeptide and/or a Dlx2 polypeptide in glial cells include, without limitation, NG2, GFAP, Olig2, CAG, EF1a, Aldh1L1, and CMV promoters.

As used herein, “operably linked” refers to positioning of a regulatory element in a vector relative to a nucleic acid in such a way as to permit or facilitate expression of the encoded polypeptide. For example, a viral vector can contain a glial-specific NG2 promoter and nucleic acid encoding a NeuroD1 polypeptide or a Dlx2 polypeptide. In this case, the NG2 promoter is operably linked to a nucleic acid encoding a NeuroD1 polypeptide or a Dlx2 polypeptide such that it drives transcription in glial cells.

Nucleic acid encoding a NeuroD1 polypeptide and/or a Dlx2 polypeptide also can be administered to a mammal using non-viral vectors. Methods of using non-viral vectors for nucleic acid delivery are described elsewhere. See, for example, Gene Therapy Protocols (Methods in Molecular Medicine), edited by Jeffrey R. Morgan, Humana Press, Totowa, N.J. (2002). For example, nucleic acid encoding a NeuroD1 polypeptide and/or a Dlx2 polypeptide can be administered to a mammal by direct injection of nucleic acid molecules (e.g., plasmids) comprising nucleic acid encoding a NeuroD1 polypeptide and/or a Dlx2 polypeptide, or by administering nucleic acid molecules complexed with lipids, polymers, or nanospheres. In some cases, a genome editing technique such as CRISPR/Cas9-mediated gene editing can be used to activate endogenous NeuroD1 and/or Dlx2 gene expression.

Nucleic acid encoding a NeuroD1 polypeptide and/or a Dlx2 polypeptide can be produced by techniques including, without limitation, common molecular cloning, polymerase chain reaction (PCR), chemical nucleic acid synthesis techniques, and combinations of such techniques. For example, PCR or RT-PCR can be used with oligonucleotide primers designed to amplify nucleic acid (e.g., genomic DNA or RNA) encoding a NeuroD1 polypeptide and/or a Dlx2 polypeptide.

In some cases, NeuroD1 polypeptides and/or Dlx2 polypeptides can be administered in addition to or in place of nucleic acid designed to express a NeuroD1 polypeptide and/or nucleic acid designed to express a Dlx2 polypeptide. For example, NeuroD1 polypeptides and/or Dlx2 polypeptides can be administered to a mammal to trigger glial cells within the brain into forming GABAergic neurons that can be functionally integrated into the brain of the living mammal.

Nucleic acid designed to express a NeuroD1 polypeptide and nucleic acid designed to express a Dlx2 polypeptide (or NeuroD1 and/or Dlx2 polypeptides) can be delivered to glial cells within the brain (e.g., glial cells within the striatum) via direct intracranial injection, intraperitoneal administration, intranasal administration, intravenous administration, or oral delivery in nanoparticles and/or drug tablets, capsules, or pills.

As described herein, nucleic acid designed to express a NeuroD1 polypeptide and nucleic acid designed to express a Dlx2 polypeptide (or NeuroD1 and/or Dlx2 polypeptides) can be administered to a mammal (e.g., a human) having Huntington's disease and used to improve the brain architecture of the Huntington's disease patient's brain such that it more closely resembles the brain architecture of a healthy human, to restore a healthy brain architecture to a Huntington's disease patient's brain, to reduce the progression of Huntington's disease, to delay the onset of Huntington's disease symptoms, to treat Huntington's disease, or to reverse the effects of Huntington's disease in the mammal. In some cases, nucleic acid designed to express a polypeptide having the amino acid sequence set forth in SEQ ID NO:1 and nucleic acid designed to express a polypeptide having the amino acid sequence set forth in SEQ ID NO:2 (or a polypeptide having the amino acid sequence set forth in SEQ ID NO:1 and/or a polypeptide having the amino acid sequence set forth in SEQ ID NO:2) can be administered to a mammal (e.g., a human) having Huntington's disease as described herein and used to improve the brain architecture of the Huntington's disease patient's brain such that it more closely resembles the brain architecture of a healthy human, to restore a healthy brain architecture to a Huntington's disease patient's brain, to reduce the progression of Huntington's disease, to delay the onset of Huntington's disease symptoms, to treat Huntington's disease, or to reverse the effects of Huntington's disease in the mammal. For example, a single adeno-associated viral vector can be designed to express a polypeptide having the amino acid sequence set forth in SEQ ID NO:1 and a polypeptide having the amino acid sequence set forth in SEQ ID NO:2, and that designed viral vector can be administered to a human having Huntington's disease to treat Huntington's disease.

In some cases, a polypeptide containing the entire amino acid sequence set forth in SEQ ID NO:1, except that the amino acid sequence contains from one to ten (e.g., ten, one to nine, two to nine, one to eight, two to eight, one to seven, one to six, one to five, one to four, one to three, two, or one) amino acid additions, deletions, substitutions, or combinations thereof, can be used. For example, nucleic acid designed to express a polypeptide containing the entire amino acid sequence set forth in SEQ ID NO:1 with one to ten amino acid additions, deletions, substitutions, or combinations thereof and nucleic acid designed to express a Dlx2 polypeptide (or the polypeptides themselves) can be designed and administered to a human having Huntington's disease to treat Huntington's disease.

In some cases, a polypeptide containing the entire amino acid sequence set forth in SEQ ID NO:2, except that the amino acid sequence contains from one to ten (e.g., ten, one to nine, two to nine, one to eight, two to eight, one to seven, one to six, one to five, one to four, one to three, two, or one) amino acid additions, deletions, substitutions, or combinations thereof, can be used. For example, nucleic acid designed to express a polypeptide containing the entire amino acid sequence set forth in SEQ ID NO:2 with one to ten amino acid additions, deletions, substitutions, or combinations thereof and nucleic acid designed to express a NeuroD1 polypeptide (or the polypeptides themselves) can be designed and administered to a human having Huntington's disease to treat Huntington's disease. In another example, nucleic acid designed to express a polypeptide containing the entire amino acid sequence set forth in SEQ ID NO:1 with one to ten amino acid additions, deletions, substitutions, or combinations thereof and nucleic acid designed to express a polypeptide containing the entire amino acid sequence set forth in SEQ ID NO:2 with one to ten amino acid additions, deletions, substitutions, or combinations thereof can be designed and administered to a human having Huntington's disease to treat Huntington's disease.

Any appropriate amino acid residue set forth in SEQ ID NO:1 and/or SEQ ID NO:2 can be deleted, and any appropriate amino acid residue (e.g., any of the 20 conventional amino acid residues or any other type of amino acid such as ornithine or citrulline) can be added to or substituted within the sequence set forth in SEQ ID NO:1 and/or SEQ ID NO:2. The majority of naturally occurring amino acids are L-amino acids, and naturally occurring polypeptides are largely comprised of L-amino acids. D-amino acids are the enantiomers of L-amino acids. In some cases, a polypeptide as provided herein can contain one or more D-amino acids. In some embodiments, a polypeptide can contain chemical structures such as ε-aminohexanoic acid; hydroxylated amino acids such as 3-hydroxyproline, 4-hydroxyproline, (5R)-5-hydroxy-L-lysine, allo-hydroxylysine, and 5-hydroxy-L-norvaline; or glycosylated amino acids such as amino acids containing monosaccharides (e.g., D-glucose, D-galactose, D-mannose, D-glucosamine, and D-galactosamine) or combinations of monosaccharides.

Amino acid substitutions can be made, in some cases, by selecting substitutions that do not differ significantly in their effect on maintaining (a) the structure of the peptide backbone in the area of the substitution, (b) the charge or hydrophobicity of the molecule at particular sites, or (c) the bulk of the side chain. For example, naturally occurring residues can be divided into groups based on side-chain properties: (1) hydrophobic amino acids (norleucine, methionine, alanine, valine, leucine, and isoleucine); (2) neutral hydrophilic amino acids (cysteine, serine, and threonine); (3) acidic amino acids (aspartic acid and glutamic acid); (4) basic amino acids (asparagine, glutamine, histidine, lysine, and arginine); (5) amino acids that influence chain orientation (glycine and proline); and (6) aromatic amino acids (tryptophan, tyrosine, and phenylalanine). Substitutions made within these groups can be considered conservative substitutions. Non-limiting examples of substitutions that can be used herein for SEQ ID NO:1 and/or SEQ ID NO:2 include, without limitation, substitution of valine for alanine, lysine for arginine, glutamine for asparagine, glutamic acid for aspartic acid, serine for cysteine, asparagine for glutamine, aspartic acid for glutamic acid, proline for glycine, arginine for histidine, leucine for isoleucine, isoleucine for leucine, arginine for lysine, leucine for methionine, leucine for phenyalanine, glycine for proline, threonine for serine, serine for threonine, tyrosine for tryptophan, phenylalanine for tyrosine, and/or leucine for valine. Further examples of conservative substitutions that can be made at any appropriate position within SEQ ID NO:1 and/or SEQ ID NO:2 are set forth in Table 1.

TABLE 1 Examples of conservative amino acid substitutions. Original Exemplary Preferred Residue substitutions substitutions Ala Val, Leu, Ile Val Arg Lys, Gln, Asn Lys Asn Gln, His, Lys, Arg Gln Asp Glu Glu Cys Ser Ser Gln Asn Asn Glu Asp Asp Gly Pro Pro His Asn, Gln, Lys, Arg Arg Ile Leu, Val, Met, Ala, Phe, Norleucine Leu Leu Norleucine, Ile, Val, Met, Ala, Phe Ile Lys Arg, Gln, Asn Arg Met Leu, Phe, Ile Leu Phe Leu, Val, Ile, Ala Leu Pro Gly Gly Ser Thr Thr Thr Ser Ser Trp Tyr Tyr Tyr Trp, Phe, Thr, Ser Phe Val Ile, Leu, Met, Phe, Ala, Norleucine Leu

In some embodiments, polypeptides can be designed to include the amino acid sequence set forth in SEQ ID NO:1 or SEQ ID NO:2 with the proviso that it includes one or more non-conservative substitutions. Non-conservative substitutions typically entail exchanging a member of one of the classes described above for a member of another class. Whether an amino acid change results in a functional polypeptide can be determined by assaying the specific activity of the polypeptide using, for example, the methods disclosed herein.

In some cases, a polypeptide having an amino acid sequence with at least 85% (e.g., 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99.0%) sequence identity to the amino acid sequence set forth in SEQ ID NO:1, provided that it includes at least one difference (e.g., at least one amino acid addition, deletion, or substitution) with respect to SEQ ID NO:1, can be used. For example, nucleic acid designed to express a polypeptide containing an amino acid sequence with between 90% and 99% sequence identity to the amino acid sequence set forth in SEQ ID NO:1 and nucleic acid designed to express a Dlx2 polypeptide (or the polypeptides themselves) can be designed and administered to a human having Huntington's disease to treat Huntington's disease.

In some cases, a polypeptide having an amino acid sequence with at least 85% (e.g., 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99.0%) sequence identity to the amino acid sequence set forth in SEQ ID NO:2, provided that it includes at least one difference (e.g., at least one amino acid addition, deletion, or substitution) with respect to SEQ ID NO:2, can be used. For example, nucleic acid designed to express a polypeptide containing an amino acid sequence with between 90% and 99% sequence identity to the amino acid sequence set forth in SEQ ID NO:2 and nucleic acid designed to express a NeuroD1 polypeptide (or the polypeptides themselves) can be designed and administered to a human having Huntington's disease to treat Huntington's disease. In another example, nucleic acid designed to express a polypeptide containing an amino acid sequence with between 90% and 99% sequence identity to the amino acid sequence set forth in SEQ ID NO:1 and nucleic acid designed to express a polypeptide containing an amino acid sequence with between 90% and 99% sequence identity to the amino acid sequence set forth in SEQ ID NO:2 (or the polypeptides themselves) can be designed and administered to a human having Huntington's disease to treat Huntington's disease.

Percent sequence identity is calculated by determining the number of matched positions in aligned amino acid sequences, dividing the number of matched positions by the total number of aligned amino acids, and multiplying by 100. A matched position refers to a position in which identical amino acids occur at the same position in aligned amino acid sequences. Percent sequence identity also can be determined for any nucleic acid sequence.

The percent sequence identity between a particular nucleic acid or amino acid sequence and a sequence referenced by a particular sequence identification number (e.g., SEQ ID NO:1 or SEQ ID NO:2) is determined as follows. First, a nucleic acid or amino acid sequence is compared to the sequence set forth in a particular sequence identification number using the BLAST 2 Sequences (Bl2seq) program from the stand-alone version of BLASTZ containing BLASTN version 2.0.14 and BLASTP version 2.0.14. This stand-alone version of BLASTZ can be obtained online at fr.com/blast or at ncbi.nlm.nih.gov. Instructions explaining how to use the Bl2seq program can be found in the readme file accompanying BLASTZ. Bl2seq performs a comparison between two sequences using either the BLASTN or BLASTP algorithm. BLASTN is used to compare nucleic acid sequences, while BLASTP is used to compare amino acid sequences. To compare two nucleic acid sequences, the options are set as follows: -i is set to a file containing the first nucleic acid sequence to be compared (e.g., C:\seq1.txt); -j is set to a file containing the second nucleic acid sequence to be compared (e.g., C:\seq2.txt); -p is set to blastn; -o is set to any desired file name (e.g., C:\output.txt); -q is set to -l; -r is set to 2; and all other options are left at their default setting. For example, the following command can be used to generate an output file containing a comparison between two sequences: C:\Bl2seq c:\seq1.txt -j c:\seq2.txt -p blastn -o c:\output.txt -q -l -r 2. To compare two amino acid sequences, the options of Bl2seq are set as follows: -i is set to a file containing the first amino acid sequence to be compared (e.g., C:\seq1.txt); -j is set to a file containing the second amino acid sequence to be compared (e.g., C:\seq2.txt); -p is set to blastp; -o is set to any desired file name (e.g., C:\output.txt); and all other options are left at their default setting. For example, the following command can be used to generate an output file containing a comparison between two amino acid sequences: C:\Bl2seq c:\seq1.txt -j c:\seq2.txt -p blastp -o c:\output.txt. If the two compared sequences share homology, then the designated output file will present those regions of homology as aligned sequences. If the two compared sequences do not share homology, then the designated output file will not present aligned sequences.

Once aligned, the number of matches is determined by counting the number of positions where an identical nucleotide or amino acid residue is presented in both sequences. The percent sequence identity is determined by dividing the number of matches by the length of the sequence set forth in the identified sequence (e.g., SEQ ID NO:1), followed by multiplying the resulting value by 100. For example, an amino acid sequence that has 340 matches when aligned with the sequence set forth in SEQ ID NO:1 is 95.5 percent identical to the sequence set forth in SEQ ID NO:1 (i.e., 340±356×100=95.5056). It is noted that the percent sequence identity value is rounded to the nearest tenth. For example, 75.11, 75.12, 75.13, and 75.14 is rounded down to 75.1, while 75.15, 75.16, 75.17, 75.18, and 75.19 is rounded up to 75.2. It also is noted that the length value will always be an integer.

The invention will be further described in the following examples, which do not limit the scope of the invention described in the claims.

EXAMPLES Example 1—In Vivo Reprogramming Glial Cells into GABAergic Neurons in the Striatum to Treat Huntington's Disease NG2 Cell Culture

As described elsewhere (Guo et al., Cell Stem Cell, 14:188-202 (2014)), mouse cortical tissue was dissected out and isolated from the brain of postnatal pups (P3-P5). Cortical cells were dissociated (0.25% trypsin-EDTA) and plated in 25 cm² flasks coated with poly-D-lysine (PDL, Sigma), and cultured in DMEM/F12 (GIBCO) with 10% fetal bovine serum (FBS, GIBCO) for 9 days, with the medium changed once every 3 days. On the ninth day, the flasks were rigorously shook, and the supernatant was collected and centrifuged to harvest NG2 cells with a small number of neurons, astrocytes, and microglial cells. The majority of astrocytes were flat and not easy to shake off the flasks. After centrifuge, cells were resuspended and seeded on PDL-coated coverslips (12 mm). The NG2 cells were maintained in serum-free DMEM medium (GIBCO) supplied with N2 supplements (STEMCELL), 10 ng/mL platelet-derived growth factor (PDGF, Invitrogen), 10 ng/mL FGF2 (Invitrogen), and 10 ng/mL EGF (Invitrogen), at 37° C. in humidified air with 5% CO2.

Retrovirus Production

The human NG2 promoter gene was subcloned from hNG2 Promoter-GLuc (GeneCopoeia) and used to replace the CAG promoter in pCAG retroviral vector, which encoded either NeuroD1 or GFP, as described elsewhere (Guo et al., Cell Stem Cell, 14:188-202 (2014)), to generate pNG2-NeuroD1-IRES-GFP or pNG2-GFP-IRES-GFP. The mouse Dlx2 cDNA was subcloned from pCAG-Dlx2-IRES-DsRed (Heinrich et al., PloS Biology, (2010)) (obtained from Dr. Magdalena Gotz) and inserted into pNG2 retroviral vector to generate pNG2-Dlx2-IRES-GFP. The E2A-Dlx2 cDNA was a PCR product from the template plasmid pCAG-Dlx2-IRES-DsRed using a 5′ primer containing an E2A peptide. This PCR product was inserted into pNG2 retroviral vector to generate pNG2-NeuroD1-E2A-Dlx2-IRES-GFP. The pCAG-NeuroD1-IRES-GFP was constructed as described elsewhere (Guo et al., Cell Stem Cell, 14:188-202 (2014)). The human ASCL1 plasmid was constructed from a PCR product using a template of the pCMV6-XL5-ASCL1 (OriGene) that was inserted into a pCAG-GFP-IRES-GFP retroviral vector (Zhao et al., J. Neurosci., (2006)) (obtained from Dr. Fred Gage) to generate pCAG-ASCL1-IRES-GFP. To package retroviral particles, the target plasmid described above were transfected into gpg helper-free human embryonic kidney (HEK) cells to generate vesicular stomatitis virus glycoprotein (VSV-G)-pseudotyped retroviruses encoding neural transcription factors. The titer of viral particles was about 10⁷ particles/mL, determined after transduction of HEK cells.

Trans-Differentiation of NG2 Cells into Neurons

Twenty-four hours after infection of mouse NG2 cells with retroviruses, the culture medium was replaced by a differentiation medium that included DMEM/F12 (GIBCO), 0.5% FBS (GIBCO), N2 supplements (GIBCO), vitamin C (VC, 5 μg/mL, Sigma), ROCK inhibitor (Y-27632, 1 μM, Selleckchem), and penicillin/streptomycin (GIBCO). To promote synaptic maturation during conversion, brain-derived neurotrophic factor (BDNF, 20 ng/mL, Invitrogen) was added to the cultures every four days. Due to the morphological change from NG2 cells to neurons during reprogramming, the empty space was filled with additional mouse astrocytes to support the functional development of converted neurons.

AAV Production

The hNG2 or hGFAP promoter was amplified by PCR and inserted into pAAV-MCS (Cell Biolab) between MluI and SacII to replace the CMV promoter. The Cre gene was obtained by PCR from hGFAP Promoter-Cre-MP-1 (Addgene) and inserted into pAAV-MCS EcoRI and SalI sites to generate pAAV-NG2::Cre and pAAV-GFAP::Cre. The Cre gene was subcloned into pAAV-MCS EcoRI and SalI sites to generate pAAV-NG2::Cre and pAAV-GFAP::Cre. To construct pAAV-FLEX-mCherry-P2A-mCherry (or pAAV-FLEX-GFP-P2A-GFP), pAAV-FLEX-NeuroD1-P2A-mCherry (or pAAV-FLEX-NeuroD1-P2A-GFP), and pAAV-FLEX-Dlx2-P2A-mCherry, the cDNAs coding NeuroD1, Dlx2, mCherry, or GFP were obtained by PCR using the retroviral constructs. The amplicons were fused with P2A-mCherry or P2A-GFP and subcloned into the pAAV-FLEX-GFP KpnI and XhoI sites. To generate pAAV-NG2::NeuroD1-P2A-GFP and pAAV-NG2::Dlx2-P2A-GFP, the proneural genes, NeuroD1 or Dlx2 fused with P2A-GFP, were subcloned into EcoRI and SalI sites after hNG2 promoter. Sequencing of the plasmid constructs was carried out to verify their identities.

AAV5 Production and Purification

Recombinant AAV5 was produced in 293AAV cells (Cell Biolabs). Briefly, polyethylenimine (PEI, linear, MW 25,000) was used for transfection of triple plasmids: the pAAV expression vector, pAAV5-RC (Cell Biolab) and pHelper (Cell Biolab). 72 hours post transfection, cells were scrapped in their medium and centrifuged, and frozen and thawed four times by placing it alternately in dry ice/ethanol and 37° C. water bath. AAV crude lysate was purified by centrifugation at 54,000 rpm for 1 hour in discontinuous iodixanol gradients with a Beckman SW55Ti rotor. The virus-containing layer was extracted, and viruses were concentrated by Millipore Amicon Ultra Centrifugal Filters. Virus titers were 1.2×10¹² GC/mL for GFAP::Cre and NG2::Cre, and 1.4×10¹² GC/mL for FLEx-NeuroD1-P2A-GFP, FLEx-NeuroD1-P2A-mCherry and FLEx-Dlx2-P2A-mCherry, and 1.6×10¹² GC/mL for FLEx-mCherry-P2A-mCherry, FLEx-GFP-P2A-GFP, NG2::NeuroD1-P2A-GFP and NG2::Dlx2-P2A-GFP were determined by QuickTiter™ AAV Quantitation Kit (Cell Biolabs).

Animals

All animals (C57/BL6 mice, 2-3 month old) were housed in a 12-hour light/dark cycle and fed with enough food and water.

Stereotaxic Viral Injection

Brain surgeries were conducted on 2-3 month-old C57/BL6 mice for AAV injection. The mice were anesthetized by injecting 20 mL/kg 2.5% Avertin (a mixture of 25 mg/mL of Tribromoethylethanol and 25 μL/ml T-amylalcohol) or ketamine/xylazine (120 mg/kg and 16 mg/kg) into the peritoneum and then placed in a stereotaxic setup. Artificial eye ointment was applied to cover the eye for protection purpose. The mice were operated with a midline scalp incision and were drilled with a hole (˜1 mm) on the skull for needle injection, with its position relative to Bregma as following: AP +0.6 mm, ML 1.7 mm, DV −2.8 mm for striatum; AP +1.8 mm, ML 2.5 mm, DV −1.8 mm for prefrontal cortex. Each mouse received an injection of AAV using a 5 μL syringe and a 34G needle. The injection volume was 2 μL, and the flow rate was controlled at 0.2 μL/minute. After viral injection, the needle was kept in place for at least five additional minutes before slowly withdrawn.

Immunocytochemistry

For brain slice immunostaining, the animals were anesthetized with 2.5% Avertin and then quickly perfused with saline (0.9% NaCl) to wash away the blood followed with 4% paraformaldehyde (PFA) to fix the brains. The brains were then taken out and postfixed in 4% PFA overnight in cold room. After fixation, the samples were cut at 35 μm coronal sections by a vibratome (Leica), washed three times by PBS, and permeabilized in 0.3% Triton X-100 in phosphate-buffered saline (PBS, pH 7.4) for one hour. For GABA staining, the permeabilized step was skipped. All brain sections were blocked in blocking buffer (2.5% normal goat serum (NGS), 2.5% normal donkey serum (NDS), and 0.1% Triton X-100 in PBS) for another hour. For cell culture immunostaining, cells were fixed in 4% PFA in PBS for 12 minutes at room temperature. After fixation, the cultures were first washed three times by PBS and then permeabilized in 0.1% Triton X-100 in PBS for 30 minutes. All samples were blocked by blocking buffer for one hour before incubation with primary antibodies.

Primary antibodies, dissolved in blocking buffer, were incubated either with brain sections or culture samples overnight in cold room. After washing three times in PBS, the samples were incubated with appropriate secondary antibodies conjugated to DyLight 488, DyLight 594, Alexa Flour 647 and Cy3 (1:1000, Jackson ImmunoResearch) for one hour at room temperature, followed by extensive washing in PBS. Coverslips were finally mounted onto a glass slide with an anti-fading mounting solution with DAPI (Invitrogen). All images were taken by a confocal microscope (Olympus FV1000). Z-stacks of confocal images were acquired and analyzed using FV10-ASW 3.0 Viewer software (Olympus).

Antibodies

The following primary antibodies were used: polyclonal anti-green fluorescent protein (GFP, chicken, 1:2000, Abcam, AB13970), polyclonal anti-glial fibrillary acidic protein (GFAP, rabbit, 1:1000, Dako, Z0334), monoclonal anti-CNPase (mouse, 1:800, Abcam, AB6319), polyclonal anti-vesicular glutamate transporter 1 (vGluT1, rabbit, 1:1000, Synaptic Systems), monoclonal anti GAD67 (mouse, 1:1000, Millipore, MAB5406), monoclonal anti GAD65 (GAD-6, mouse, 1:1000, Developmental Studies Hybridoma Bank, Iowa City), monoclonal anti-NG2 (mouse, 1:600, Abcam, AB50009), polyclonal anti-Iba1 (rabbit, 1:1000, Wako, 019-19741), polyclonal anti-NeuN (rabbit, 1:1000, Millipore, ABN78), monoclonal anti-βIII tubulin (Tuj1, mouse, 1:1000, COVANCE, MMS-435P), polyclonal anti-γ-aminobutyric acid (GABA, rabbit, 1:2000, Sigma, A2052), polyclonal anti-Red Fluorescent Protein (RFP, rat, 1:1500, antibody-online, ABIN334653; and rabbit, 1:1000, Rockland, 600-401-379), polyclonal anti-T-box brain 1 (Tbr1, 1:800, rabbit, Abcam, AB31940), monoclonal anti-Ctip2 (Rat, 1:1000, Abcam, AB18465), monoclonal anti-NeuroD1 (mouse, 1:1000, Abcam, AB60704), polyclonal anti-Dlx2 (rabbit, 1:600, Abcam, AB30339), polyclonal anti-Doublecortin (DCX, rabbit, 1:1000, Abcam, AB18723), monoclonal anti-Parvalbumin (PV, mouse, 1:1000, sigma, P3088), polyclonal anti-Calretinin (CR, goat, 1:1500, Millipore, AB1550), monoclonal anti-Somatostatin (SST, rat, 1:300, Millipore, MAB354), polyclonal anti-Cholecystokinin (Catalog Nos. 26-33) (CCK-8, rabbit, 1:2000, sigma, C2581), polyclonal anti-Neuropeptide Y (NPY, rabbit, 1:2000, Abcam, AB30914), and polyclonal anti-Dopamine- and cAMP-regulated neuronal phosphoprotein (DARP-32, rabbit, 1:1500, Millipore, AB10518).

Image Analysis

Cell counts were performed by taking images of several randomly chosen views per coverslip or brain slice and analyzed by Image J software. Data were presented as mean±SEM. Student's t-test (paired or unpaired) was used for statistical analysis. *P<0.05, **P<0.01, ***P<0.001.

Electrophysiology Patch-Clamp Recordings in Cell Cultures

Whole-cell recordings were conducted using Multiclamp 700A patch-clamp amplifier (Molecular Devices, Palo Alto, Calif.) as described elsewhere (Guo et al., Cell Stem Cell, 14:188-202 (2014)). The recording chamber was continuously perfused with a bath solution consisting of 128 mM NaCl, 30 mM glucose, 25 mM HEPES, 5 mM KCl, 1 mM MgCl₂, and 2 mM CaCl₂. The bath solution, with an osmolarity at 315-325 mOsm/L, was adjusted to pH 7.3 by NaOH. Patch pipettes were pulled from borosilicate glass (3-5 MΩ) and filled with an internal solution consisting of 125 mM KGluconate, 10 mM KCl, 10 mM Tris-phosphocreatine, 10 mM HEPES, 5 mM EGTA, 4 mM MgATP, and 0.5 mM Na₂GTP (pH 7.3, adjusted with KOH) for recording action potentials. A different internal solution composed of 135 mM CsGluconate, 5 mM EGTA, 10 mM HEPES, 10 mM Tris-phosphocreatine, 4 mM MgATP, and 0.5 mM Na₂GTP (pH 7.3, adjusted with KOH) was used for recording GABAergic synaptic responses (IPSCs). The series resistance was typically 15-30 MΩ and not compensated to avoid increased noise associated with compensation. GABA_(A) receptor antagonist bicuculline (Bic) was applied through a gravity-driven drug delivery system (VC-6, Warner Hamden, Conn.). For voltage-clamp experiments, the membrane potential was typically held at −20 or 0 mV for recording upward IPSCs. Data were acquired using pClamp 9 software (Molecular Devices, Palo Alto, Calif.), sampled at 10 kHz, and filtered at 1 kHz. Action potentials were analyzed using pClamp 9 Clampfit software, and spontaneous synaptic events were analyzed using MiniAnalysis software (Synaptosoft, Decator, Ga.). All experiments were performed at room temperature.

Brain slice recordings Brain slices were prepared at 1-1.5 month after AAV injection, and cut at 300 μm thick coronal sections with a Leica vibratome in ice-cold cutting solution (75 mM sucrose, 87 mM NaCl, 0.5 mM CaCl₂, 2.5 mM KCl, 7 mM MgCl₂, 1.25 mM NaH₂PO₄, 25 mM NaHCO₃, and 20 mM glucose). Slices were incubated in artificial cerebral spinal fluid (ACSF) containing: 119 mM NaCl, 2.5 mM KCl, 26 mM NaHCO₃, 1.25 mM NaH₂PO₄, 2.5 mM CaCl₂, 1.3 mM MgCl₂, and 10 mM glucose, and constantly bubbled with 95% O₂ and 5% CO2 at 32-33° C. for 30 minutes. Brain slices were then transferred to a chamber at room temperature to recover for one hour. The recording chamber was set at 32-33° C. for all recordings. Whole-cell recordings were conducted using a pipette solution consisting of 135 mM K-Gluconate, 5 mM Na-phosphocreatine, 10 mM KCl, 2 mM EGTA, 10 mM HEPES, 4 mM MgATP, and 0.5 mM Na₂GTP (pH 7.3, adjusted with KOH, 290 mOsm/L). Pipette resistance was typically 4-6 MΩ, and series resistance was around 20-40 MΩ. The membrane potential was held at −70 mV when recording spontaneous events. Data were collected using pClamp 9 software (Molecular Devices, Palo Alto, Calif.), sampled at 10 kHz, and filtered at 1 kHz, then analyzed with pClamp 9 Clampfit and MiniAnalysis software (Synaptosoft, Decator, Ga.).

Reprogramming Cultured NG2 Cells into Functional GABAergic Neurons

NG2 cells are glial progenitor cells that mainly produce oligodendrocytes in both physiological and pathological conditions (Kang et al., Neuron, 68:668-681 (2010); Buffo et al., Proc. Natl. Acad. Sci. USA, 105:3581-3586 (2008); and Nishiyama et al., Nat. Rev. Neurosci., 10:9-22 (2009)). The following was performed to test whether Dlx2, a transcription factor critical for GABAergic fate determination, can reprogram NG2 cells into GABAergic neurons.

First, primary culture of mouse NG2 cells were used to test Dlx2 reprogramming capability. The cell cultures were enriched with NG2 cells, as demonstrated by successful infection with retrovirus expressing GFP under the control of human NG2 promoter (NG2::GFP-IRES-GFP). The majority of cells were immunopositive for oligodendrocyte marker CNPase (FIGS. 1A and 2D; 71.5±5.8% CNPase+; n=4 batches). In contrast, only about 10% of cells were GFAP-positive astrocytes, and none were NeuN-positive neurons (FIG. 2). The NG2 cultures were infected with retrovirus expressing Dlx2 under the control of NG2 promoter (NG2::Dlx2-IRES-GFP). Many Dlx2-infected NG2 cells became immunopositive for NeuN, a neuronal marker, after one week of infection (FIG. 1B; 57.1±5.1% NeuN+; n=5 batches; 7 DPI), suggesting that Dlx2 is capable of reprogramming NG2 cells into neuronal cells. Importantly, these NG2-reprogrammed neurons induced by Dlx2 exhibited many GABAergic synapses (FIG. 1C), but few glutamatergic synapses (FIG. 3), suggesting that NG2 cells are likely converted into GABAergic neurons but not glutamatergic neurons. To corroborate the immunostaining results, the functional properties of NG2-converted neurons were examined by patch-clamp recordings after two weeks of Dlx2 infection. Significant GABAergic synaptic events were detected in NG2-converted neurons (FIG. 1D; IPSC frequency, 0.7±0.2 Hz; amplitude, 19.6±3.7 pA; n=7; 14 DPI), which were mostly blocked by GABA_(A) receptor antagonist bicuculline (BIC, 20 μM), confirming that NG2-converted neurons are GABAergic. Thus, Dlx2 can reprogram cultured NG2 cells into functional GABAergic neurons.

Co-Expression of NeuroD1 and Dlx2 Increases the Conversion Efficiency

While Dlx2 alone was capable of reprogramming NG2 cells into GABAergic neurons, the reprogramming efficiency was not very high. To increase the reprogramming efficiency, other neural transcription factors such as Ascl1 (also known as Mash1; Vierbuchen et al., Nature, 463:1035-1041 (2010) and Bertrand et al., Nat. Rev. Neurosci., 3:517-530 (2002)) and NeuroD1 (Guo et al., Cell Stem Cell, 14:188-202 (2014) and Kuwabara et al., Nat. Neurosci., 12:1097-1105 (2009)), as well as combinations of Dlx2 with Ascl1 and NeuroD1, were tested (FIG. 4). Among all the combinations tested, co-overexpression of NeuroD1 and Dlx2 together generated the most number of GABAergic neurons (FIG. 4A-B, n=3 repeats, 14 DPI). Functional assay were used with electrophysiological recordings to analyze GABAergic events among different groups (FIG. 4C). The highest frequency of GABAergic events was also detected in the NeuroD1+Dlx2 group (FIG. 3D, n=3 repeats, 21 DPI), consistent with the immunostaining results.

A polycistronic retroviral vector expressing NeuroD1 and Dlx2 together under the control of NG2 promoter (NG2::NeuroD1-E2A-Dlx2-IRES-GFP) was constructed. The new retrovirus simultaneously expressing NeuroD1 and Dlx2 generated many more neurons than Dlx2 alone (FIG. 1E). Quantitatively, the Tuj1⁺ neurons in the NeuroD1+Dlx2 group (8.2±1.0 cells/0.1 mm²) were 3-fold more than that infected by Dlx2 alone (2.3±0.3 cells/0.1 mm²; n=4 repeats; 12-14 DPI) (FIG. 1G). The conversion efficiency also increased from 57.2±8.8% by Dlx2 alone to 94.9±2.1% by NeuroD1+Dlx2 together. GAD67 staining was performed, which confirmed that the NeuroD1+Dlx2 converted neurons were mainly GABAergic (FIGS. 1F and 1H; NeuroD1+Dlx2, 8.8±0.9 cells/0.1 mm²; Dlx2, 1.4±0.2 cells/0.1 mm²; n=4 repeats; 19-21 DPI). Glutamatergic neurons were <3% after NeuroD1+Dlx2 conversion as revealed by VGlut1 staining (2.5±1.2%; n=4 repeats; 19-21 DPI). Therefore, NeuroD1 significantly facilitates Dlx2-mediated reprogramming of NG2 cells into functional GABAergic neurons.

NG2-Converted GABAergic Neurons have Specific Subtype Properties

GABAergic interneurons have been classified into many subtypes according to specific protein expression markers, such as but not limited to calretinin (CR), parvalbumin (PV), somatostatin (SST), neuropeptide Y (NPY), and cholecystokinin (CCK) (Kepecs & Fishell, Nature, 505:318-326 (2014); Kawaguchi & Kondo, J. Neurocytol., 31:277-287 (2002); and Nat. Rev. Neurosci., 9:557-568 (2008)). Therefore, a series of immunostaining was performed with a variety of GABAergic markers to determine what specific subtypes of GABAergic neurons were converted from NG2 cells (FIG. 5). Interestingly, nearly 90% of NG2-converted neurons were immunopositive for CR or SST (FIG. 5A-B), over 50% were PV-positive (FIG. 5C), and less cells positive for CCK or NPY (FIG. 5D-E). Quantitative results were shown in FIG. 5F (CR, 91.8±5.6%; SST, 89.3±5.5%; PV, 53.9±7.8%; CCK8, 23.3±9.9%; NPY, 14.9±3.9%; n=3 batches; 19-21 DPI). GABAergic neurons also can be characterized according to their firing patterns, such as fast-spiking action potentials found in parvalbumin neurons (Markram et al., Nat. Rev. Neurosci., 5:793-807 (2004)). When analyzed the NG2-converted neurons with patch-clamp recordings, after NeuroD1-Dlx2 retroviral infection, some neurons fired fast-spiking like action potentials whereas others fired lower frequency action potentials (FIG. 5G-H, n=15 cells), supporting that NG2-converted neurons are a mixture of PV and non-PV interneurons. These results demonstrate that Dlx2 together with NeuroD1 can efficiently convert cultured NG2 cells into mature GABAergic neurons with a variety of subtypes, including PV, CR, and SST neurons.

In Vivo Reprogramming of NG2 Cells into Functional GABAergic Neurons

After reprogramming cultured NG2 cells into GABAergic neurons in vitro, the following was performed to examine whether NG2 cells in the mouse brain in vivo also can be converted into GABAergic interneurons. As demonstrated elsewhere (Guo et al., Cell Stem Cell, 14:188-202 (2014)), the in vivo reprogramming efficiency induced by retroviruses is not very high. In an attempt to overcome this, recombinant adeno-associated virus (serotype 5, rAAV5) were made for in vivo reprogramming. Among different serotypes of rAAV, rAAV5 was chosen for the majority of the experiments because it prefers to infect glial cells over neuronal cells (Howard et al., Virology, 372:24-34 (2008) and Markakis et al., Mol. Ther., 18:588-593 (2010)). In order to specifically target NG2 cells and achieve stable transgene insertion, a Cre-FLEx (flip-excision) system, which includes a vector expressing Cre recombinase under the control of human NG2 promoter (NG2::Cre) and a vector with Cre-mediated FLEx switch of the inverted coding sequence of NeuroD1-P2A-mCherry or Dlx2-P2A-mCherry (FIG. 6A; Schnutgen et al., Nat. Biotechnol., 21:562-565 (2003) and Atasoy et al., J. Neurosci., 28:7025-7030 (2008)), was developed. The Cre-FLEx rAAV system was designed to allow Cre expression in NG2 cells, which in turn will activate the transcription of NeuroD1 or Dlx2 together with mCherry. For a control experiment, rAAV-NG2::Cre together with rAAV-FLEx-mCherry were first injected into the mouse striatum, a brain region enriched with GABAergic interneurons. Control rAAV (NG2::Cre and FLEx-mCherry) successfully infected NG2 cells in the striatum as revealed by NG2 immunostaining (FIG. 6B; 66.7±11.1% infected cells were NG2+; n=3 animals). Interestingly, after injecting rAAV-NG2::Cre plus rAAV-FLEx-NeuroD1 and rAAV-FLEx-Dlx2 into the striatum (FIG. 6C, Cre+NeuroD1+Dlx2), many infected cells were observed with neuron-like morphology and immunopositive for NeuN (FIG. 6D; 80.8±1.9% mCherry-labeled cells were also NeuN positive, n=4 animals; 21 DPI). To investigate the time course of NG2-neuron conversion, the percentage of NG2 cells versus neurons among the total number of viral infected cells (mCherry positive) from 4 to 21 days post viral injection (Cre+NeuroD1+Dlx2) was examined. Interestingly, a steady decrease of NG2 cells accompanied with a steady increase of Tuj1⁺ or NeuN⁺ neurons was observed during this conversion period (FIG. 6E), suggesting that NG2 cells are gradually converted into neurons.

To further demonstrate direct conversion of NG2 glia into neurons, AAV5 vectors were constructed using NG2 promoter to directly drive the expression of NeuroD1 or Dlx2 (NG2::NeuroD1-P2A-GFP+NG2::Dlx2-P2A-GFP), without using Cre/FLEx system. Compared to GFP expression alone under the control of NG2 promoter (NG2::GFP), many more neurons (NeuN-positive) were detected after expressing NeuroD1+Dlx2 (FIG. 7). Therefore, NeuroD1 and Dlx2 together can efficiently reprogram NG2 cells into neuronal cells in the mouse brain in vivo. It is noted that the human NG2 promoter is not a specific promoter.

The following was performed to investigate whether NG2 cells in the striatum were converted into GABAergic neurons, as found in NG2 cell cultures. GABA and GAD67 immunostaining were performed, which confirmed that many NG2-converted neurons were indeed GABAergic neurons (FIG. 6F-G; 61.2±3.6% infected cells were GABA+; n=4 animals; 21 DPI). To test whether NG2-converted neurons were functionally connected with other neurons, brain slice recordings were performed at 1-1.5 months after NeuroD1+Dlx2 viral injection. The NG2-converted neurons exhibited robust spontaneous synaptic events (FIG. 6H; frequency, 6.8±1.3 Hz; amplitude, 13.3±0.9 pA; HP=−70 mV; n=16; 30-45 DPI). These results demonstrate that striatal NG2 cells can be reprogrammed into functional GABAergic neurons in situ after ectopic expression of NeuroD1 and Dlx2.

Regional Influence on the Subtypes of GABAergic Neurons

The subtypes of in vivo NG2-converted GABAergic neurons were further characterized using a series of GABAergic neuron markers (FIG. 8). In the striatum, medium spiny neurons are projecting neurons, not interneurons, but they are GABAergic neurons (Gangarossa et al., Front Neural Circuits, 7:22 (2013)). A significant proportion of NG2-converted neurons in the striatum were DARPP32-positive medium spiny neurons (FIG. 8A, 40.6±2.8%). Because medium spiny neurons are mostly sensitive to the toxic effects in Huntington's disease, this in vivo reprogramming method can be used as a new therapy to treat Huntington's disease by regenerating medium spiny neurons from internal glial cells. There also was a significant number of NG2-converted neurons immunopositive for PV (FIG. 8B), but rarely positive for CCK8 (FIG. 8C-D). Quantitatively, about 19.9±1.9% of NG2-converted neurons were PV⁺ GABAergic neurons and 9.3±2.3% neurons were SST⁺ (FIG. 8F, n=3 animals), consistent with previous report that PV and SST neurons are the two major subtypes of interneurons in the striatum (Marin et al., J. Neurosci., 20:6063-6076 (2000)). Functional analysis with brain slice recordings revealed that some NG2-converted neurons were capable of firing fast-spiking action potentials (FIG. 8G), indicating that they are likely PV interneurons. These results demonstrate that striatal NG2 cells can be reprogrammed in situ into DARPP32⁺ and PV⁺ GABAergic neurons by ectopic expression of Dlx2 and NeuroD1 together.

The following was performed to investigate whether NG2 cells in different brain regions are reprogrammed into different subtypes of GABAergic neurons when using the same transcription factors NeuroD1+Dlx2. The same rAAV used in the striatum (NG2::Cre+FLEx-NeuroD1+FLEx-Dlx2) was injected into mouse prefrontal cortex, where the subtypes of GABAergic neurons are different from the striatum. As a control experiment, expression of mCherry alone (NG2::Cre+FLEx-mCherry) infected mainly NG2 cells in the prefrontal cortex (FIG. 9A, 77.5±1.3% infected cells were NG2+; n=3 animals; see, also, FIG. 11). In contrast, ectopic expression of Dlx2 and NeuroD1 in the NG2 cells of prefrontal cortex reprogrammed most of the NG2 cells into neurons (FIG. 9B-C; 72±1.1% mCherry⁺ cells were also NeuN⁺; n=3 animals; 21 DPI). Importantly, the majority of NG2-converted neurons also were immunopositive for GABA or GAD67 (FIG. 9D-E; 69.3±8% mCherry⁺ cells were also GAD67⁺; n=3 animals; 21 DPI), suggesting that they were GABAergic neurons. Moreover, brain slice recordings showed robust spontaneous synaptic events in the cortical NG2-converted neurons (FIG. 9F; frequency, 4.5±1.7 Hz; amplitude, 14.6±2.1 pA; n=7; 30-45 DPI), indicating that the NG2-converted neurons were functionally integrated into local neural circuits. Therefore, similar to striatal NG2 cells, cortical NG2 cells can be reprogrammed into functional GABAergic neurons by ectopic expression of Dlx2 and NeuroD1 together. However, different from the striatum where a large proportion of NG2-reprogrammed cells were DARPP32-positive medium spiny neurons, cortical NG2 cells were found to be reprogrammed into mainly CCK8 (39.7±2.2%, n=3 animals) or PV-positive neurons (26.3±3.4%, n=3 animals) (FIGS. 10B, 10D, and 10F), while fewer neurons were SST- or NPY-positive (FIGS. 10A and 10C). No DARPP32-positive cells were detected among cortical NG2-converted cell population (FIG. 10E-F). Patch-clamp recordings showed that cortical NG2-converted neurons also were capable of firing fast-spiking action potentials or lower frequency action potentials (FIG. 10G), confirming a mixture of different subtypes of GABAergic neurons converted from cortical NG2 cells. These results demonstrate that striatal and cortical NG2 cells can be reprogrammed into different subtypes of GABAergic neurons after expressing the same transcription factors Dlx2 and NeuroD1, indicating that either intrinsic lineage trace inside the regional glial cells or local environmental factors may regulate the fate choice of glia-neuron conversion.

Example 2—Regenerating Medium Spiny Neurons to Treat Huntington's Disease

A Huntington's disease (HD) mouse model, R6/2 transgenic mice, which carries 120 CAG repeats from human HD gene and exhibit disease onset at about 8-12 weeks of age is obtained. NeuroD1+Dlx2 AAV is injected into these HD mice at age of 6, 8, 10, and 12 weeks old to confirm that the in vivo reprogramming technology described herein extends the life of HD mice and improves the function of these HD mice. Injecting NeuroD1 and Dlx2 into the striatum of HD mice can have the ability to regenerate functional GABAergic neurons including medium spiny neurons, which in turn can increase the life span and rescue at least some of the functional deficits of HD mice.

Example 3—In Situ Conversion of Glial Cells into GABAergic Neurons Inside Brains

AAV5 viral vectors were produced to express NeuroD1 and Dlx2. The AAV5 expressing NeuroD1 and Dlx2 viral vectors were injected into R6/2 mice, a mouse model for Huntington's disease. Following injection of AAV5 expressing NeuroD1 and Dlx2, astrocytes of the striatum generated new neurons in the Huntington's disease model R6/2 mice (FIGS. 14A-F). Most of the NeuroD1+Dlx2 converted neurons were GABAergic neurons (FIGS. 15A-B).

In addition to AAV5 viral vectors, the capability of AAV9 viral vectors for in vivo cell conversion was confirmed. An AAV9 viral vector was used to express NeuroD1 and Dlx2 in astrocytes under the control of astrocyte promoter GFAP. The AAV9 viral vectors expressing NeuroD1 and Dlx2 were able to convert astrocytes into neurons (FIG. 16A-D). Furthermore, immunostaining with a GABAergic neuron marker (GAD67) demonstrated that the majority of NeuroD1/Dlx2-converted neurons were GABAergic neurons (FIG. 17A-B). Among the GABAergic neurons, some of the NeuroD1/Dlx2-converted neurons were DARPP32-positive medium spiny neurons, which are often vulnerable in Huntington's disease. These results demonstrate that adeno-associated viral vectors such as AAV5 or AAV9 can be designed to express NeuroD1 and Dlx2 and that such designed vectors can be used to convert glial cells into GABAergic neurons, including DARPP32-positive medium spiny neurons, thereby treating Huntington's disease.

Other Embodiments

It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims. 

1. (canceled)
 2. A composition for converting glial cells to GABAergic neurons in a striatum of a living mammal's brain, wherein said composition comprises a nucleic acid vector comprising a nucleic acid sequence encoding a neurogenic differentiation 1 (NeuroD1) polypeptide and a nucleic acid sequence encoding a distal-less homeobox 2 (Dlx2) polypeptide.
 3. The composition of claim 2, wherein said nucleic acid vector is a viral vector.
 4. The composition of claim 3, wherein said viral vector is an adeno-associated viral vector.
 5. The composition of claim 2, wherein said nucleic acid sequence encoding said NeuroD1 polypeptide or said nucleic acid sequence encoding said Dlx2 polypeptide is operably linked to a promoter sequence, wherein said promoter sequence is a constitutive promoter sequence.
 6. The composition of claim 5, wherein said constitutive promoter sequence is selected from the group consisting of a NG2 promoter sequence, a GFAP promoter sequence, an EF1a promoter sequence, a CMV promoter sequence, an Aldh1L1 promoter sequence, and a CAG promoter sequence.
 7. The composition of claim 2, wherein said nucleic acid sequence encoding said NeuroD1 polypeptide or said nucleic acid sequence encoding said Dlx2 polypeptide is operably linked to a promoter sequence, wherein said promoter sequence is a glial-specific promoter sequence.
 8. The composition of claim 7, wherein said glial-specific promoter sequence is selected from the group consisting of a NG2 promoter sequence, a GFAP promoter sequence, an A1dh1L1 promoter sequence, and an Olig2 promoter sequence.
 9. The composition of claim 2, wherein said mammal is diagnosed with Huntington's disease.
 10. The composition of claim 9, wherein said mammal is a human.
 11. The composition of claim 2, wherein said glial cells are NG2 glial cells.
 12. The composition of claim 2, wherein said GABAergic neurons are DARPP32-positive.
 13. The composition of claim 2, wherein said NeuroD1 polypeptide is a human NeuroD1 polypeptide comprising an amino acid sequence of SEQ ID NO:
 1. 14. The composition of claim 2, wherein said Dlx2 polypeptide is a human Dlx2 peptide comprising an amino acid sequence of SEQ ID NO:
 2. 